The present invention relates to a wide-band microwave voltage-controlled oscillator (VCO).
A common and convenient way to configure a VCO is by using a varactor to change the resonance point of a tuned circuit, and connecting this tuned circuit to the gate of an FET which supplies the necessary gain for oscillation. However, as is well known in the art, the gate must also be biased to an appropriate operating point, to provide adequate gain and power output from the FET. While many circuits to set the gate operating point are known, they commonly have the difficulty that the elements (lumped or even distributed) used to set the DC gate bias have their own high frequency resonances. Where it is desired to configure a microwave VCO, such spurious resonances may commonly fall within the band of interest, and thus interfere with proper operation of the VCO.
Thus, it is an object of the present invention to provide a VCO wherein bias is provided to set the operating point without producing additional spurious resonances in the tuned resonance section of the VCO.
In particular, when it is desired to configure a very wide band VCO, the chance that a spurious resonance occurring in the DC return path which is conventionally used to set gate bias may fall within the bandwidth of interest is very large.
Thus, it is a particular object of the present invention to provide a gate-bias circuit for a very wide band VCO.
Normally, the frequency tuning range of varactor-based VCO is much narrower than the capacitance range of the tuning varactor, due to the inherent and parasitic reactance characteristics of FETs and other components of the VCO. In particular, an extremely wide-range varactor (having a capacitance ratio of a decade or more) is needed if the frequency range of the VCO is to remotely approach one octave. In particular, to exploit the tuning range provided by the varactor in the tuned gate-control circuit, it is highly desirable to minimize the parasitic reactances which narrow the frequency-tuning ratio of a VCO which can be attained based on a given capacitance-tuning ratio of the varactor included in the VCO.
Thus, it is a further object of the present invention to provide a VCO having a frequency-tuning range which is a substantial fraction of the capacitance-tuning range of a varactor included in the VCO.
Thus, it is an object of the present invention to provide a VCO having a tuning range of 1.5 to 1 or larger at microwave frequencies. It is a further object of the present invention to provide a VCO having a tuning range of 1.3 to 1 or better at microwave frequencies above 5 GHz.
It is a further object of the present invention to provide a microwave VCO having a tuning range of an octave or more.
It is a further object of the present invention to provide a monolithic microwave VCO having a tuning range of 1.5 to 1 or larger.